This invention relates to the alkylation of hydrocarbons such as aromatics and paraffins to produce useful chemicals and motor fuel. This invention specifically relates to a method and apparatus for alkylation using a transport reactor.
Hydrocarbon alkylation is widely used in the petroleum refining and petrochemical industries to produce a variety of useful acyclic and cyclic hydrocarbon products which are consumed in motor fuel, plastics, detergent precursors, and petrochemical feedstocks. Alkylation processes generally involve the alkylation of an alkylation substrate with an alkylating agent. The alkylation substrate is an aromatic hydrocarbon such as benzene if the process produces ethylbenzene, cumene, or linear alkyl benzenes. If the process produces motor fuels such as gasoline, the alkylation substrate may be a branched paraffinic hydrocarbon having from 4 to 6 carbon atoms. The alkylating agent is typically an olefinic hydrocarbon containing from 2 to about 20 carbon atoms, depending on the desired product of the process.
Much of the installed base of alkylation capacity uses liquid phase hydrofluoric acid, generally referred to as HF, as the catalyst. The use of HF in these applications has a long record of highly dependable and safe operation. However, the potential damage from an unintentional release of any sizable quantity of HF and the need to safely dispose of some byproducts produced in the process has led to an increasing demand for alkylation process technology which does not employ liquid phase HF as the catalyst.
Numerous solid alkylation catalysts have been described in the open literature. However, these catalysts appear to suffer from unacceptably high deactivation rates when employed at commercially feasible conditions. While some catalysts have a sufficiently useful lifetime to allow the performance of alkylation, the rapid change in activity results in a change in product composition and also requires the periodic regeneration of the catalyst with the accompanying removal of the reaction zone from operation. It is very desirable to provide a continuous process for alkylation which is not subjected to periodic reaction zone stoppages or variation in the product stream composition.
Transport reactors are commonly used in hydrocarbon processing. In a transport reactor, the catalyst bed as a whole moves. Thus, a transport reactor can be contrasted with a fixed bed catalytic reactor and with an ebullated bed catalytic reactor. In a fixed bed reactor the catalyst particles do not move, and in an ebullated bed reactor the catalyst particles are suspended in a fluid but the settling velocity of the catalyst particles balances the fluid upflow velocity so that the catalyst bed as a whole does not move. Although it is generally the case that the direction of catalyst flow through a transport reactor is upward, the direction may also be downward, horizontal, a direction that is intermediate between vertical and horizontal, or a combination of these directions.
When the direction of catalyst flow through a transport reactor is upward, the transport reactor is often called a riser-reactor. Riser-reactors are commonly used in hydrocarbon processing, such as fluidized catalytic cracking and more recently in fluidized solid bed motor fuel alkylation. In a common arrangement, a fluid hydrocarbon reactant engages a solid hydrocarbon conversion catalyst at the bottom of a riser-reactor and transports the catalyst in a fluidized state up the riser-reactor. During the ascent through the riser-reactor, the catalyst promotes certain desired conversion reactions among the reactants in order to produce desired products. A stream of catalyst and hydrocarbon products, by-products, and unreacted reactants if any discharges from the top of the riser-reactor into a separation zone. The hydrocarbons and the catalyst disengage in the separation zone, with the hydrocarbons being withdrawn overhead for recovery and the catalyst dropping by gravity to the bottom of the separation zone. Despite some deactivation that may have occurred to the catalyst in the riser-reactor, some of the catalyst that collects at the bottom of the separation zone usually has enough residual activity that it can be reused in the riser-reactor without first being withdrawn from the separation zone for regeneration. Such still active catalyst is recirculated through a recirculation conduit from the bottom of the separation zone to the bottom of the riser-reactor, where the catalyst contacts reactants again.
Several methods are used for controlling the introduction of reactants and for controlling the recirculation of catalyst to the bottom of the riser-reactor. For example, one method is shown in a motor fuel alkylation process in U.S. Pat. No. 5,489,732 (Zhang et al.). Isoparaffins and olefins are introduced into the bottom of the riser-reactor, and the flow of catalyst through a single recirculation conduit to the bottom of the riser-reactor is controlled by several means including slide valves, other types of valves, lock hoppers, fluid flow control (reverse flow of liquid), screw conveyors, and L-valves. This patent also teaches that one reactant, isobutane, can also be introduced into the recirculation conduit for the purpose of flushing by-product hydrogen from the recirculating catalyst. This method, however, is not suitable for withdrawing catalyst symmetrically or uniformly from the bottom of the separation zone, if the bed of catalyst in the bottom of the separation zone is not totally fluidized in the axial direction, i.e., it is a moving packed bed or a bed that is merely at incipient fluidization. In these types of beds, catalyst that is below the angle of repose from the opening to the recirculation pipe remains stagnant, which leads to inefficient use of the separation zone. Areas of stagnant catalyst can lead to operational difficulties if, because of an upset or disruption, the stagnant catalyst breaks loose, enters the recirculation pipe, and enters the riser-reactor. Another method that uses a spout-fluid bed with a draft tube is shown in the article by H. Littman et al. entitled xe2x80x9cFluid Flow Pattern and Solids Circulation Rate in a Liquid Phase Spout-Fluid Bed with Draft Tube,xe2x80x9d The Canadian Journal of Chemical Engineering, Vol. 70, October 1992, pp. 895-904. This method provides poor control of the fraction of the total flow rate of reactants to the bottom of the draft tube that would flow through the draft tube compared to that fraction which would flow in reverse flow through the annular bed around the draft tube and would effectively bypass the draft tube. Moreover, this method provides poor control of the catalyst flow rate to the bottom of the draft tube, once the geometry around the bottom of the draft tube is fixed.
Accordingly, there is a need for a method and an apparatus that is suitable for use in a transport reactor process that uniformly or symmetrically withdraws catalyst from the separation zone which separates the transport reactor effluent, that uniformly or symmetrically controls the flow of reactants to the bottom of the transport reactor, and that controls the flow of catalyst from the separation zone to the transport reactor.
This invention is a novel method and apparatus for alkylating an alkylation substrate with an alkylating agent using solid catalyst particles in a fluidized transport reactor. The effluent of the transport reactor passes to a separation zone, which separates the product alkylate from the solid catalyst particles. The solid catalyst particles recirculate from the separation zone to the transport reactor through two or more recirculation conduits. The recirculation rate of catalyst particles through each recirculation conduit is regulated by a fluid-controlled valve that uses the alkylation substrate as the regulating fluid. Each fluid-controlled valve discharges catalyst through a conduit into the transport reactor, so that a single, common transport reactor is fed by all of the fluid-controlled valves. This invention is particularly applicable to transport reactors that are riser-reactors.
This method and apparatus have numerous advantages over the prior art. By using two or more recirculation conduits rather than a single conduit, this invention can help to ensure uniform residence time distribution of catalyst particles and to minimize areas of stagnant catalyst particles in the separation zone. This, in turn, helps prevent unexpected changes in riser-reactor performance that can occur when catalyst particles of varying activity enter the riser-reactor. This invention also helps ensure that the upward flow of reactants is through the riser-reactor rather than through the recirculation pipes, which helps prevent bypassing of the riser-reactor by the reactants. In addition, by having all fluid-controlled valves feed into a single, common riser-reactor rather than to a separate riser-reactor for each fluid-controlled valve, this invention is simpler to build and operate.
It is an objective of this invention to provide an alkylation process which does not employ liquid phase HF as the catalyst. It is a further objective of the subject invention to provide an alkylation process which utilizes a solid catalyst. It is a specific objective of the invention to provide a solid catalyst alkylation process for the alkylation of liquid hydrocarbons for the production of motor fuel blending hydrocarbons.
Accordingly, in one embodiment, this invention is a process for the alkylation of an alkylation substrate with an alkylating agent. An alkylating agent, a feed stream comprising an alkylation substrate, a first recirculation stream, and a second recirculation stream pass to an alkylation transport reactor. The first recirculation stream and the second recirculation stream each comprise catalyst particles and the alkylation substrate. In the alkylation transport reactor, the alkylating agent alkylates the alkylation substrate in the presence of a fluidized bed of catalyst particles at alkylation conditions, thereby producing alkylate. A transport reactor effluent stream comprising alkylate and catalyst particles passes from the alkylation transport reactor to a separation zone, where the transport reactor effluent stream is separated. Catalyst particles and a product stream comprising alkylate are recovered from the separation zone. A first portion of the catalyst particles recovered from the separation zone pass to a first fluid-controlled valve. The flow of alkylation substrate into the first fluid-controlled valve is regulated to produce the first recirculation stream and to deliver catalyst particles to the alkylation transport reactor. A second portion of the catalyst particles recovered from the separation zone pass to a second fluid-controlled valve, and the flow of alkylation substrate into the second fluid-controlled valve is regulated to produce the second recirculation stream and to deliver catalyst particles to the alkylation transport reactor.
In a more detailed embodiment, this invention is a process for the alkylation of isobutane with butenes. A feed stream comprising isobutane, a first recirculation stream comprising catalyst particles and isobutane, and a second recirculation stream comprising catalyst particles and isobutane passing into a substantially vertical alkylation riser-reactor. The riser-reactor has a bottom portion and a top portion that is oriented above the bottom portion, and the feed stream, the first recirculation stream, and the second recirculation stream enter the bottom portion of the riser-reactor. Butenes pass to the bottom portion of the riser-reactor and to at least three intermediate portions of the riser-reactor, where the at least three intermediate portions are located between the bottom portion and the top portion of the riser-reactor. The butenes alkylate isobutane in the presence of a fluidized bed of catalyst particles at alkylation conditions in the riser-reactor, and produce alkylate. A riser-reactor effluent stream discharges from the top portion of the riser-reactor to a separation zone. The riser-reactor effluent stream comprises alkylate and catalyst particles that are partially deactivated. In the separation zone, the riser-reactor effluent stream is separated into a product stream comprising alkylate and into catalyst particles that are partially deactivated. The product stream is recovered from the process, and the catalyst particles pass downwardly in the separation zone. In a lower portion of the separation zone, a dense fluidized bed forms. The dense fluidized bed contains catalyst particles that are partially deactivated. In the dense fluidized bed, isobutane and hydrogen contact the catalyst particles at reactivation conditions sufficient to at least partially reactivate the catalyst particles. After being contacted with isobutane and hydrogen, a first aliquot portion of catalyst particles passes from the separation zone to a substantially vertical first recirculation conduit. In the first recirculation conduit, the catalyst particles form a first moving packed bed. The catalyst particles pass downwardly through the first moving packed bed to a first fluid-controlled valve. The flow of isobutane into the first fluid-controlled valve is regulated to produce the first recirculation stream and to deliver catalyst particles to a horizontal first feeder conduit. The first recirculation stream is conveyed through the first feeder conduit and to the bottom portion of the riser-reactor. A second aliquot portion of catalyst particles, after being contacted with isobutane and hydrogen, passes from the separation zone to a substantially vertical second recirculation conduit, where the catalyst particles form a second moving packed bed. The catalyst particles pass downwardly through the second moving packed bed to a second fluid-controlled valve. The flow of isobutane into the second fluid-controlled valve is regulated to produce the second recirculation stream and to deliver catalyst particles to a horizontal second feeder conduit. The second recirculation stream is conveyed through the second feeder conduit and to the bottom portion of the riser-reactor.
In another embodiment, this invention is an apparatus for alkylating liquid hydrocarbons using solid catalyst particles. A substantially vertical transport reactor has a reactor inlet and a reactor outlet. The reactor inlet and the reactor outlet in part define a reactor space for maintaining a bed of solid catalyst particles and liquid hydrocarbons. A means for discharging solid catalyst particles and liquid hydrocarbons from the transport reactor communicates with the reactor outlet of the transport reactor. A means for disengaging solid catalyst particles and liquid hydrocarbons communicates with the means for discharging. A vessel is in communication with the means for disengaging. The vessel has a means for receiving solid catalyst particles from the means for disengaging. In addition, the vessel has a vessel outlet that in part defines a vessel space for maintaining a bed of solid catalyst particles. A first recirculation conduit, which extends in a substantially vertical direction, has first conduit inlet that is in communication with the vessel outlet for receiving solid catalyst particles. The first recirculation conduit also has a first conduit outlet. A first valve conduit has a first valve inlet in communication with the first conduit outlet for receiving solid catalyst particles and a first valve outlet in communication with the reactor inlet for discharging solid catalyst particles. The first valve conduit also has a first means for introducing liquid hydrocarbons at a controlled rate into the first valve conduit between the first valve inlet and the first valve outlet. A second recirculation conduit, which extends in a substantially vertical direction, has a second conduit inlet that is in communication with the vessel outlet for receiving solid catalyst particles. The second recirculation conduit also has a second conduit outlet. A second valve conduit has a second valve inlet in communication with the second conduit outlet for receiving solid catalyst particles and a second valve outlet in communication with the reactor inlet for discharging solid catalyst particles. The second valve conduit also has a second means for introducing liquid hydrocarbons at a controlled rate into the second valve conduit between the second valve inlet and the second valve outlet. Means are provided for introducing liquid hydrocarbons into the reactor inlet.